When a circuit is designed/drawn, the assumption is that the circuit can actually be produced as drawn. However, process effects cause distortion of the “drawn” image as it is printed on the wafer. One of the effects is line end shortening (“LES”), also known as line end pullback; it is a significant issue in microlithography. LES is a function of a variety of causes, including annularity (i.e., the exposure technique using apertures of various sizes and combinations of apertures), feature size, intensity flux (i.e., exposure in a given area), exposure, numerical aperture (“NA”), resist features (e.g., type, thickness, etc.), exposure wavelength, and focus (i.e., whether or not the image is in focus). LES usually occurs at the end of a feature in a lithography process (e.g., a line or space, rectangular in shape where the length is assumed to be longer than the width) and is visible in either polarity (i.e., either a positive or negative image of the feature). When LES occurs, the circuit may not be complete because all of its required contacts either have not been made at all or barely overlap which results in failures due to contact resistance issues.
Current LES quantification methods are non-electrical, time-consuming and expensive. For example, scanning electron microscope (“SEM”) cross-sections are expensive and destroy the product. SEM cross-sections take a great deal of time to perform, thereby severely curtailing the number of measurement samples that can be made. Additionally, the accuracy of SEM cross-sections as applied to LES quantification is only about 5%. Another quantification method is SEM top-down. This method is also time-consuming and usually requires use of a manufacturing tool for extended periods of time in order to generate data.
It is therefore desirable to provide a solution that minimizes product loss and reduces the use of manufacturing tools, such as SEMs, in metrology. It is also desirable to provide a solution that enables the accumulation of enough data from each wafer/lot yield to improve data confidence. The present invention provides this in some embodiments by utilizing a pattern of incrementally length-modified conductive paths that each include pairs of conductors, each pair connected to respective contacts that are physically separated but electrically connected to one another. The conductors can vary in length by a constant increment, beginning with a length that results in a significant overlap at the contacts to a length that results in a significant underlap at the contacts. Resistance measurements of each conductive path can be made until a change either to or from an “open” occurs; this is the point from which, using the constant increment, the LES can be characterized.